Citalopram is a well-known antidepressant drug that has now been on the market for some years and has the following structure: 
It is a selective, centrally acting serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor, which has further been disclosed to show effects in the treatment of dementia and cerebrovascular disorders, cf. EP-A-474580.
Citalopram was first disclosed in DE 2,657,013, corresponding to U.S. Pat. No. 4,136,193. This patent publication i.a. outlines a process for preparation of citalopram from the corresponding 5-bromo-derivative by reaction with cuprous cyanide in a suitable solvent. Further processes for the preparation of citalopram by exchange of 5-halogen or CF3—(CF2)n—SO2—O—, n being 0-8, with cyano are disclosed in WO 0011926 and WO 0013648.
Other processes involve:                Conversion of a 5-amido or 5-ester group to a 5-cyano group (WO 9819513)        Conversion of a 5-amino group to a 5-cyano group (WO 9819512)        Conversion of a 5-formyl group to a 5-cyano group (WO 9900548)        Conversion of a 5-oxazolinyl or 5-thiazolinyl group to a 5-cyano group (WO 0023431)        
It has turned out that it is difficult to manufacture citalopram in the required quality. The processes of DE 2,657,013, WO 0011926 and WO 0013648 comprising exchange of 5-halogen with cyano as described above have now been found to give some high molecular weight impurities including dimeric reaction products in unacceptable amounts. These impurities are difficult to remove by usual working up procedures leading to extensive and expensive purification processes.
Thus a process for the preparation of citalopram, in which impurities formed during the cyanide exchange reaction, i.e. the exchange of 5-halogen or the like with 5-cyano, are removed, is needed in order to obtain a commercially attractive manufacture of citalopram.
It has now been found that these high molecular reaction impurities may be removed by a film distillation process.